\documentclass{article}


%%-----------PGF/TIKZ
\usepackage{pgf}
\usepackage{tikz}
\usetikzlibrary{arrows,shapes,shapes.arrows,positioning}

\usepackage{palatino}

% Use preview.sty to crop the page
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{pgfpicture}
\setlength\PreviewBorder{0pt}

\begin{document}

\begin{figure}
\begin{tikzpicture}
    [
    node distance=8mm and 20mm,
    scale=1,
    block/.style ={
        rectangle,
        draw=black,
	    thick,
        text width=9em,
        fill=gray!20, 
        text centered 
    },
    decision/.style = {
        diamond,
        shape aspect=1.2,
        inner sep=-2pt,
        draw=black,
        thick,
        text width=6em,
        fill=gray!20, 
        text centered
    },
    trap/.style = {
        trapezium,
        trapezium left angle=60, 
        trapezium right angle=120,
        draw=black,
	    thick,
        text width=8em,
        fill=gray!20, 
        text centered 
    },
    phantom/.style = {
    	    inner sep=0,
		outer sep=-.7pt,
		node distance=10mm
	},
	arrow/.style ={
		draw,
		-stealth',
		thick
	}
    ]

	\node(begin) [block] {Begin with initial conditions at time interval $k=1$};
    \node(assume) [trap,below=of begin] {Compute initial flow};
    \node(rk) [trap,below=of assume] {Compute head at time $k+1$};
    \node(endsim) [decision,below=of rk] {Last Time Step?};
    \node(friction) [trap,right=of begin] {Compute frictional losses};
    \node(flow) [trap,below=of friction] {Re-Compute flow};
    \node(converge) [decision,node distance=6.5mm and 20mm,below=of flow] {Flow rate converged?};
    \node(end) [block,right=of endsim,node distance=10.3mm and 20mm,below=of converge] {End Simulation};
    \node(phantom1) [phantom, left=of endsim] {};
    \node(phantom2) [phantom, right=of assume] {};
    \node(phantom3) [phantom, right=of converge] {};

   
    
    \path[arrow] (begin) -- (assume);
    \path[arrow] (assume) -- (phantom2) |- (friction);
    \path[arrow] (endsim) -- node[above]{no} (phantom1) |- (assume);
    \path[arrow] (rk) -- (endsim);
    \path[arrow] (endsim) -- node[above]{yes} (end);
    \path[arrow] (friction) -- (flow);
    \path[arrow] (flow) -- (converge);
    \path[arrow] (converge) -- (end);
    \path[arrow] (converge) -- node[above]{no} (rk);
    \path[arrow] (converge) -- node[above]{yes} (phantom3) |- (friction);


	
\end{tikzpicture}
\end{figure}
\end{document}
